1. Field of the Invention
The present invention relates to a photographing lens system that is suitable for use in an imaging apparatus such as a digital camera, and especially for a ‘mirrorless’ interchangeable-lens camera in which no split mirror is provided for an optical finder.
2. Description of Related Art
In recent years there has been a demand for photographing lens systems used in an imaging apparatus such as a digital camera to be further miniaturization while having a high optical quality. Photographing lens systems which are configured of a positive front lens group and a positive rear lens group, in that order from the object side, are known in the art (see Japanese Unexamined Patent Publication Nos. 2009-86221, 2009-198855, 2010-191069 and 2004-101880). Since each of these photographing lens systems are for use in a single-lens reflex (SLR) camera, in order to avoid the split-mirror of the optical finder interfering with the photographing lens system between the photographing lens system and the imaging surface (of an image sensor), a negative lens element is typically provided closest to the object side within the front lens group in order to attain a long backfocus with respect to the focal length.
However, in the photographing lens system of Japanese Unexamined Patent Publication No. 2004-101880, since there area large number of lens elements, i.e., five lens elements, in the rear lens group, this becomes a cause of enlargement of the entire optical system. Furthermore, since a negative lens element is provided closest to the image side within the rear lens group so that the peripheral light rays are greatly diverged, spherical aberration, coma and astigmatism increase, and telecentricity is also lost.
In the photographing lens systems disclosed in Japanese Unexamined Patent Publication Nos. 2010-191069 and 2004-101880, since the refractive power balance between the front lens group and the rear lens group is unsuitable, large amounts of aberrations such as spherical aberration, coma and astigmatism occur, and the optical quality is deteriorated. For example, in the photographing lens system disclosed in Japanese Unexamined Patent Publication No. 2010-191069, the refractive power of the rear lens group is too strong compared to that of the front lens group, so that astigmatism occurs. In the photographing lens system disclosed in Japanese Unexamined Patent Publication No. 2004-101880, the refractive power of the front lens group is too strong compared to that of the rear lens group, so that large amounts of spherical aberration and coma occur.
Furthermore, in each of the photographing lens systems disclosed in the above-mentioned Japanese Unexamined Patent Publication Nos. 2009-86221, 2009-198855, 2010-191069 and 2004-101880, the backfocus (the distance from the imaging surface (imaging plane) to the surface on the image side of the lens element provided closest to the image side of the photographing lens system) is too long, and if utilized in a mirrorless interchangeable-lens camera, the photographing lens system becomes undesirably large. On the other hand, in a photographing lens system that is used in a mirrorless interchangeable-lens camera, since it is necessary to attain a certain amount of length between the imaging surface (imaging plane) and the surface of the negative lens element provided closest to the image side within the photographing lens system in order to reduce the sharpness of any images of dust, etc., that may have adhered to the lens element provided closest to the image side when the interchanging of the lens (barrel) while achieving miniaturization, it is not a matter of merely reducing the backfocus to an excessive degree.